Yes, and you can often check in before the other person arrives. However, when you engage in these activities dishonestly, for the sole purpose of getting certs you otherwise wouldn't get, you are telling lies to steal. It's very clear.

The T&Cs indicate it is unacceptable. Marriott's response indicated it is unacceptable. Marriott is the sole arbiter for MRs, so there is no getting around it is unacceptable.

This is different than the ten credits for CY meeting rooms, as there is no rule in the T&Cs requiring you actually have the meeting. There is a clear, straightforward rule indicating you cannot stay using someone else's MR# and you cannot get points/credits for those stays. The meeting room thing is a loophole that Marriott could choose to close, but doesn't. The cert stealing is clearly different, as it involves telling lies and intentionally breaking clear, written rules.